1. Field of the Invention
This invention relates generally to telecommunications, and, more particularly, to providing an unbalanced circuit with a built-in ring circuitry for a line driver.
2. Description of the Related Art
In communications systems, particularly telephony such as a Plain Old Telephone System (POTS), it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire, bi-directional communication channel. A line card generally connects the subscriber station to the central switching office. The functions of the line card include supplying talk battery, performing wake-up sequences of circuits to allow communications to take place, and the like. Voltage signals are processed and conditioned when being driven onto telecommunication lines.
POTS was designed primarily for voice communication, and thus provides an inadequate data transmission rate for many modern applications. To meet the demand for high-speed communication, designers have sought innovative and cost-effective solutions that would take advantage of the existing network infrastructure. Several technological solutions proposed in the telecommunications industry use the existing network of telephone wires. A promising one of these technologies is the Digital Subscriber Line (xDSL or DSL) technology.
xDSL is making the existing network of telephone lines more robust and versatile. Once considered virtually unusable for broadband communications, an ordinary twisted pair equipped with DSL interfaces can transmit video, television, and very high-speed data. The fact that more than six hundred million telephone lines exist around the world is a compelling reason for these lines to be used as the primary transmission conduits for at least several more decades. Because DSL utilizes telephone wiring already installed in virtually every home and business in the world, it has been embraced by many as one of the more promising and viable options.
There are now at least three popular versions of DSL technology, namely Asymmetrical Digital Subscriber Line (ADSL), Very High-Speed Digital Subscriber Line (VDSL), and Symmetric Digital Subscriber Line (SDSL). Although each technology is generally directed at different types of users, they all share certain characteristics. For example, all four DSL systems utilize the existing, ubiquitous telephone wiring infrastructure, deliver greater bandwidth, and operate by employing special digital signal processing. Because the aforementioned technologies are well known in the art, they will not be described in detail herein.
DSL and POTS technologies can co-exist in one line (e.g., also referred to as a “subscriber line”). Traditional analog voice band interfaces use the same frequency band, 0-4 Kilohertz (KHz), as telephone service, thereby preventing concurrent voice and data use. A DSL interface, on the other hand, operates at frequencies above the voice channels, from 25 KHz to 1.1 Megahertz (MHz). Thus, a single DSL line is capable of offering simultaneous channels for voice and data. It should be noted that the standards derivatives of ADSL are still in definition as of this writing, and therefore are subject to change.
DSL systems use digital signal processing (DSP) to increase throughput and signal quality through common copper telephone wire. It provides a downstream data transfer rate from the DSL Point-of-Presence (POP) to the subscriber location at speeds of up to 1.5 megabits per second (MBPS). The transfer rate of 1.5 MBPS, for instance, is fifty times faster than a conventional 28.8 kilobits per second (KBPS) transfer rate typically found in conventional POTS systems.
DSL systems generally employ a signal detection system that monitors the telephone line for communication requests. More specifically, the line card in the central office polls the telephone line to detect any communication requests from a DSL data transceiver, such as a DSL modem, located at a subscriber station. There are multiple types of signals that are received and transmitted over multiple signal paths during telecommunication operation. Many times it is advantageous to transmit signals in a voltage format, so as to reduce transmission power consumption.
Executing a ringing cycle in telephones has always been one of the more challenging and costly functions for Subscriber Line Interface Equipment. The traditional methods employed by telephone exchanges generally have been based on a high voltage AC ringing signal with a large DC offset voltage. The AC signal was used to ring a mechanical bell and needed to provide enough power (current and voltage) to the ringer. The DC offset was used to detect ring trip, i.e., a subscriber picking up the telephone, and to turn off the ringing signal (for safety reasons as well as to switch from the ring mode to the talk mode).
The DC offset voltage is used to detect ring trip. When the phone is on the hook and ringing, there is no DC path and the DC current is zero. When the phone is off the hook, AC and DC current flows so the traditional method of detecting that the telephone is off the hook is to sense the DC current and compare it to a threshold. The AC signal is generally filtered or canceled in order to provide the ability to measure the DC current. Generally, the larger the DC current, the easier this task is. The DC offset has traditionally been −48 Volts (the battery voltage) because it is relatively easy to generate and provides enough DC current to perform an efficient ring trip detection.
Ringing functions have been implemented in an unbalanced manner, which refers to applying a ringing waveform to one of the two leads (Tip lead or Ring lead), but not to both. One of the most common ringing method calls for providing battery backed ringing on the Ring lead (AC plus −48 Volts DC offset) and holding the Tip lead near ground. The other configuration provides for reversing battery ringing where the Tip lead has a −48 Volts DC offset and the AC signal is placed on the ring lead.
The unbalanced ringing signal typically swings from near −200V to near +100V for battery backed ringing. This 300V differential has proven to be very difficult for integrated circuit (IC) technologies. Providing IC devices that can withstand 300 Volts generally proves to be costly and inefficient. Silicon providers have tried to eliminate usage ringing relays by providing a ringing signal through devices in the line card. However, due to high voltage requirements, no practical application has been able to implement unbalanced ringing through a device in a line card. Instead, the electronic solutions have been based on a technique called balanced ringing. In the balanced ringing solution, the AC signal is applied as in-phase signals and/or 180 degrees out of phase signals to the Tip and Ring leads. In this manner, the peak AC signal can approach the total supply voltage, whereas in an unbalanced solution, the total supply must be greater than the peak-to-peak signal. In order to supply 85 Volts(rms), which has about a 125 Volts peak signal, an unbalanced ringing device in the line card requires a supply voltage greater than 250V. However, the supply voltage must also support the DC offset. An unbalanced system does not require any more voltage as long as the DC offset is less than the AC peak signal. In contrast, a balanced system requires a supply that is greater than the AC peak plus the DC offset, thereby creating an enormous design and operation burden.
Silicon technologies in the range of 150 to 170 Volts are generally being used to build ringing devices. In order to support high AC signals, the DC offset has been reduced in these solutions. For instance, if a 150 Volt supply is used, and a ringing signal of 125 Volt peak is required, there are 25 Volts left for the device-overhead and the DC offset. This forces the usage of a low DC offset to perform Ring trip detection and requires costly filtering due to the low DC offset. Therefore, it is desirable to provide an offset of close to 50 Volts, which would require a minimum supply voltage of 185 Volts. Assuming 3% supply tolerance, the supply voltage range would have to be 191 Volts (+/−5 Volts) or 197 Volts maximum. The silicon used to produce the line card devices would have to be able to handle over 200 Volts to take into account transient and surges on the power supply. The state-of-the-art technology in the industry generally does not provide a device that can handle this high voltage. Additionally, the balanced ringing device generally dissipates more power than the ring generator due to the DC supplies and overhead voltage. The ringing device also requires a more complex protection circuit than standard devices, which can increase cost.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.